Circuits for reading or amplifying signals of capacitive sensors such as a MEMS microphone usually include very high impedance input terminal(s). In such circuits, biasing may also be needed to define a DC operating point for the MEMS microphone. The biasing may use an input component with some characteristic (i.e., linearity and signal symmetry respect to the voltage reference) as well as very high impedance. For example, it may be desirable to design a biasing circuit for a MEMs-based microphone, with impedance values in excess of one GOhm, or even in excess of 100 GOhm.
Commonly proposed biasing solutions use transistors operating in the off-region to supply the required high impedance values. While a high impedance value can in fact be realized, the resultant impedance value may be sensitive to changes in the type of manufacturing Process, operating Voltage, and ambient Temperature (PVT).